Recently, the personal computer is no more limited to the desk top type and the ones getting more and more popular are of portable note book type and palmtop type which are then required to be connected to the internet for data transmission. Accordingly, the wireless internet ensues. The conventional wireless transmission is completed by the radio wave, whose application, however, is linited. The radio wave will penetrate through the wall or the partitioning medium so that radio networks in different rooms will interfere with each other. IEEE 802.11 provides a good nonn in this respect and allows the user to use the ISM channel without a license but has various limitations in order to avoid the interference between different networks. To comply with these limitations, however, not only the hardware cost is increased, but also the telecommunication performance is reduced, which explains why the infrared is proposed to serve as the medium for wireless transmission.
There are advantages by using the infrared as the medium for wireless transmission. Firstly, the radio networks in different rooms will not interfere with each other and it has an excellent security or is free from eavesdropping since the infrared cannot penetrate through the wall. Secondly, the infrared has a rather wide usable bandwidth and there is no rule to regulate its use at present or in the foreseeable future. Nevertheless, there are limitations for using the infrared to serve as the medium for wireless transmission. Firstly, the infrared or the transmission will be stopped or obstructed by the indoor furnishings. Secondly, there are a great many of infrared sources in the natural environment. For example, all of the incandescent lamp, the fluorescent lamp and the natural light include the infrared component are sources of primary noises. Accordingly, the directionless indoor infrared wireless transmission becomes an emphasis to be developed.
In a directionless indoor infrared wireless transmission, the infrared is oimnidirectionally transmitted in order that the receiver can receive an enough energy through the reflection of indoor furnishings, floor and ceiling to demodulate the signal by a proper signal to noise ratio. In 1979, it is first proposed a 125 Kbit diffusion-type infrared wireless transmission system to open a new era for infrared wireless transmission. Thereafter, 1Mbit and 2Mbit systems are subsequently announced. Owing to a vigorous demand on high-speed data transmission, there already are researches on even higher speed system.
A brief introduction to the pulse position modulation (PPM) used for infrared transmission system will be given here. An N-ary PPM divides a symbol into N slots and determines the slot address where the pulse is by the value of the data it transmits. FIGS. 1A and 1B show two kinds of 4-ary PPM where a symbol has 4 slots respectively encoded as (00, 01, 11, 10). FIG. 1A is a full slot mode and FIG. 1B is a half slot mode, in which the encoded slot is called signal slot, e.g. 01.
The data are transmitted by way of packet including a preamble sequence, a training sequence and a data sequence. The preamble sequence consists of a series of 00 symbols ended with a symbol 10, by which the receiver can determine whether the transmitter is transmitting the signal to obtain the preliminary synchronization. The training sequence provides a basis by which the receiver can judge the telecommunication channel characteristics with which the receiver can avoid the channel influence through the equalizer. The data sequence includes data to be transmitted.
In the preamble sequence, the receiver is preliminarily synchronized. It requires a timing recovery circuit (TRC) to maintain the sampling synchronization upon transmitting training sequence and data sequence. Generally speaking, a TRC can be completed by a delay locked loop (DLP). FIG. 2 is a block diagram showing an infrared receiver using a DLP. Initially, the infrared signal is amplified by a preamplifier and then quantized by an analog-to-digital converter. Thereafter, the preamble sequence detector checks whether there are continuous preamble sequence signals of fixed period in the received signal. If yes, a signal will be outputted to actuate the phase detector which will check to determine whether the sampled phase has a leading or a lagging time base. If it is lagging (or leading), the delay line will be notified to reduce (or increase) the delay time in order to be in phase with the input signal.
There are advantages and disadvantages in using the DLP as the basic structure of the infrared receiver. Advantages include reducing the over-sampling multiple of the analog-to-digital converter, which enables the later digital processing module to be manipulated in a lower speed to further minimize the power consumption. Disadvantages include being difficult to converge owing to a very large noise, which will result in an interrupted reception through a sampling timing drift. Further, the conventional timing recovery mechanism using the locked loop to soundly maintain the timing stability in a relatively longer period requires a relatively long period of time to lock the phase. Since the infrared wireless transmission uses a packet structure belonging to a transmission of burst mode, it does not require a stability in a long period of time but is to be designed into one having a wider loop bandwidth in order to quickly lock the phase. Such design, however, will have a poor stability and is easily interfered by the noise. Further, the indoor infrared transmission environment is a very bad telecommunication channel so that the phase locked loop (PLL) design becomes more complicated. Accordingly, it would appear that PLL is not a good choice.
As mentioned, the indoor infrared transmmision data is transmitted in packets each of which only has about 1 thousand of symbols. In the situation of a general frequency error, the accumulated phase error will not become too large. In an example of 100 PPM frequency error, the accumulated phase error of a packet of 1 thousand of symbols only occupies 0.1 symbol period of time. In a symbol having four time slots and having a sampling speed being 8 times that of the symbol frequency where a sampling period equals to 0.125 symbol period, the accumulated phase angle error cannot even change one sampling time base. In this situation, there still is a sample located within the active time slot. In addition, if the channel noise is too large, there will be a phase jitter upon phase detection to result in a detection error affecting the accuracy of locked phase.
It is therefore tried by the applicant to deal with the above situation encountered by the prior art.